Devices known as modems (MOdulator-DEModulators) have been used for many years to connect computer terminals and other such devices to ordinary telephone lines. Modems were developed so that digital information could be easily transmitted from one location to another distant location via the telephone line. The telephone line is capable of supporting analog signals generally in the audible frequency range and the modem is a means for modulating those signals using the digital information, coupling the modulated "carrier" to the telephone line, and demodulating signals received from other locations to recover digital information. Modems are well known in the art.
In the past, two methods were normally used to transmit data via telephone lines using modems. First, a telephone line might be dedicated to the transmission of data and would have a modem connected at each of its ends. No other signals, e.g., telephone voice signals, would be transmitted by this "dedicated" line. Second, a telephone line might be used to transmit data signals and voice signals and this might be done alternately or simultaneously. A modem and a ordinary telephone would be connected at each end of the line and would have access to the line. The dedicated line approach is more costly and, in some instances, is not practical, for example, where the number of telephone lines available is low. Therefore, the shared line is preferred in many cases.
One arrangement for sharing a telephone line between a telephone and a modem is shown in U.S. Pat. Nos. 4,524,244 and 4,578,537, both to Faggin, et al. Those patents contain virtually the same disclosure and show a digital and voice telecommunication device. A cross-point matrix comprises a combination of switches which connect the telephone line with telephone equipment and, alternatively, to a modem for data transmission and reception. In a first position, the matrix connects the telephone to the line to provide normal audio communications and connects the modem to the line to detect incoming signals. In a second position, the matrix disconnects the telephone from the line and connects the modem to allow transmission and reception of data signals. The matrix is under the control of a telephone control and computer and is normally in its first position. When it is desired to transmit data via the modem and the telephone line, an operator signals the computer and the computer switches the matrix from its first position to its second position, disconnecting the telephone from the line and providing connections for data transfer. If, while the matrix is in its first position, a carrier signal is detected on the telephone line (indicating that data is to be received by the modem), the computer automatically switches the matrix to its second position. When data transfer is complete, the computer automatically returns the matrix to its first position. In the system shown by Faggin, an incoming carrier signal will cause the computer to interrupt an ongoing voice communication on the telephone and, if an operator instructs the computer to transmit data, the computer will, likewise, interrupt any voice communications.
U.S. Pat. Nos. 4,302,629 and 4,330,687 to Foulkes, et al., show use of a non-loaded telephone line for simultaneous transmission of digital data and base band telephone signals. The telephone is connected to the "tip" and "ring" telephone wires through an audio or voice frequency filter which prevents the transmitted and received digital data signals from reaching the telephone and prevents spurious high frequency signals generated by the telephone from reaching the non-loaded loops. The modem includes a transmitter and receiver which transmits and receives pairs of frequency-shift keyed (FSK) signals. The FSK signals are chosen to lie outside the normal range of voice or audio frequencies and thus will not be passed by the audio filter to the phone. Thus, data transmission by means of the modem and voice communications by means of the telephone can occur simultaneously along the telephone line. Because of the filtering, the telephone cannot be used to monitor the transmission and reception of data by the modem.